Method of preparing organoboron compounds



e 7 2,925,441 I METHOD OEPREPARING oRGANonoRoN Y COMPOUNDS 'Herbert C. Brown, West Lafayette, Ind. I

No Drawing. Application-October so, 1958' Serial No. 770,618

.10 Claims.. (Cl. 260-6065) This invention relates toth e preparation of organoboron compounds and more particularly to the preparation of organoboron compounds having the formula R B in which R is a saturated hydrocarbon radicalisuch as an alkyl radical. Q

Hurd (Iour. Am. Chem. Soc., 70, 2053 (1948)) has reported that gaseous diborane maybe reacted with a large excess of olefinic hydrocarbons in sealed tubes at' 100 C. for extended periods of time. Thus, amixture of triisobutylboron and tri-t-butylboron was obtained by the reaction'of isobutylene and diborane in a sealed Ufl Sta ,Pa jw Patented res. 1e, 1 seo suitable low temperatures of equimolar quantities of'alu- 'minum borohydride and the complexin g agent, namely; any ether, tertiary amine or thioether.--Preterably, the complexingagent'is an ether, tertiary amine or thioether which is liquid at room temperatures and has a boiling" point substantially'difterent from that of the organobo'ron compound to be prepared. Illustrativecomplexingagents are diethyl, dipropyl; diisopropyl ethe rs anisole, tetrahydrofuran, dimethylether of diethyleneglycoL' thecofi responding thioethers and tertiary-amines,such as-trimethyl, triethyl, tripropylamine, dimethyl'aniline, ""pyri dine, N-ethyl-rnorpholine, etc. These complexes'are'sob I uble'in an excess of the complexing agent and,' in general,

aresoluble in saturated parafiinic, saturated alicyclic and" aromatic hydrocarbon solvents. Stable chlorinatedand brominated aromatic solvents, such as chlorobenzene and bromobeniene, are also suitable. The compl't'exing-gi'oup is.easily"displaced from these complexes""and, conse quently, if a given complex is dissolved inanother cont- These complexes ofaluminum borohydride are far less tube at 100 C. for 24 hours. Reaction of ethylene with diborane at 100 C. for 96 hours produced tri-ethylboron. R. S. Brokaw and R. N. Pease (Jour. Am. Chem. Soc., 72, 323 7 (1950); ibid.,' 72 5263 (1950))have reported that gaseous olefins, such as ethylene, propylene and 1-butene, react with 'aluminum borohydride at elevatedtemperatures to form trialkylborons along with mixed alkylaluminum hydrides. Thus, the reaction of ethylene with aluminum borohydride at 140 C. is postulated to be Aluminum borohydride is a volatile liquid, B3. 45 C., which is exceeding difiicult and hazardous to' handle. Small amounts ofthe liquid or vapor in contact-with the atmosphere result in violent explosions. Thus, 'the publication by Gaylord, ReductionwithComplex Metal Hydrides, Interscience Publishers, New York'city,,1956," on page 26 states Aluminum borohydrideis an extremely hazardous material. The vapor or 'the compoundignites spontaneously on exposure to air containing only traces i of. moisture. The same publication onpage 103 states The extreme reactivity of the compound, including spontaneous ignition on exposure to moist air and the explosive reaction with water, preclude any widespread utilization of reagent in organic reductions.

The present invention is based upon the discovery plexing agent, an equilibrium-mixture'oftwo diflierent complexes isobtained.

volatile than aluminum borohydride itself and aretar mole of aluminum' borohydride.

In the preparation of..organobor0n compoundsby the ,meth odfof the invention it is' preferred'to 'asso'cia'telthef reactants, namely the unsaturated organiccompound am;

the complex aluminum borohydride in a liquid carrier wihch maybe an inert non-solvent for the .corhplex'or an. inertsolventtherefor, such as a hydrocarbon or'stable haloaromatic solvent, or a' liquid capable of formingi'oiief of the above mentioned complexes with aluminum boro hydride." In many instances the complex of aluminum: borohydride isa liquid and may serve at least in part as the liquid carrier. Similarly, an organoboron compoundobtained from a previous run may serve at'least in part" as theliquid carrier. The' organobor on compound can that complexes of aluminum borohydride with ethers,

tertiary amines, or thioethers react with remarkable ease withunsaturated organic compounds, .such as olefins, to I form organoboron compounds. The reaction is complete in a few minutes at room temperature. In fact, the vigor of the reaction is such. thatthe olefin must be added slowlyto the reaction mixture in order to avoid a too rapid reaction with its concurrent development of heat. The reactions are illustrated by the' following equations:

The complexes of aluminum borohydride may. be form d y we l nq n, P 9 b t inte act n t berecovered fr'om the reaction liquor byv either of two procedures. By one procedure, thereaction liquorv is]. permittedto stand and the by-product aluminum hydride, complex precipitates and can be separated by filtration, "or decantation. The org'anoboron compound can be covered from the resulting liquor by distillation (By, .th second procedure, water maybe added to the" reaction liquor to destroy the aluminum hydride and the organoqf boron compound can be recovered from theresult in a liquor by distillation.

vThe aluminum hydride complex readily; f diborane at room temperature to form the corresponding complex of aluminum borohydride as illustrated'by the following equation:

Consequently, it is possible to carryout a two-stage procg fess in which the aluminum borohydride complex is re- 'acted with the unsaturated organic compound 'inone stage and. then the by-product aluminum hydride complex is' removed from the reaction liquor as previously described and reacted with, diborane in a liquid carriertg form the. complex of aluminum borohydride. Y

If desired, in the practice of the invention the complex of aluminum borohydride may be formed concurrently w1th the reaction of the complex with the unsaturatedorganiccompound. 'Thus, a very small amount of'the aluminum borohydride complex may be introduced initially-into the reaction vesselalong with the unsaturated organic compound in the liquid carrier. amount of complex reacts to form small amounts of the Qrganoboron compound and the complex of aluminum hydride. Diborane is introduced to convert the aluminum hydride complex to the-complex f aluminum borohydride. Diborane is introduced continuously and this cycle -.is repeated. until the reaction with the unsaturated organic -compound is complete. In placed the small amount of aluminum borohydride complex used, initially, aisrnall amount of aluminum borohydride or a material which is readily converted to aluminum borohydride by diborane may-be used if a small amount of a complexing agenLispr-esent in the reaction vessel. Examples of such materials are aluminum hydride, aluminum alkyls, mixed allcylaluminum halides and the metallic aluminum hydrides such as lithium or magnesium aluminum hydrides and their al lryl'derivatives. v

The invention particularly contemplates the conversion of olefins, .such as l-pentene, Z-pentene, 1hexene, 1- octene, diisobutylene, cyclopentene, cyclohexene, styrene, p fl-diphenylethylene, etc., to organoboron compounds having the formula R 13 in which R represents a saturated hydrocarbon-radical. However, the invention is applicable broadly for conversion to organoboron compounds of other unsaturated organic compounds including dienes such as butadiene and cyclohexadiene, acetylenes such as l-hexyne and Z-hexyne, and substituted olefins such as mnitrostyrene, nitroethylene, allyldimethylamine, vinyl pyridine,allylethylether, vinylbutylether and allylethyl' sulfide. The unsaturated organic compound used preferably should not contain a substituent which is reduced by the aluminum borohydride complex.

The term .unsatured as used herein is intended to refer to'organic compounds which owe their unsaturation to the presence of an .olefinic double bond or an acetylenic triple bond although such compounds may also contain an' aromatic ring. Conversely, the term saturated as used herein is not intended to exclude simple aromatic compounds or aromatics having one or more aliphatic or alicyclic radicals. Aromatic rings do not add bromine at room temperature whereas the other derivatives react readily.

The, invention is illustrated further by the following examples.

Preparation of tri-n-pentylboron in diethylether In a dry one liter flask was placed 0.33 mole of aluminum borohydride-diethyl ether complex in 400 ml. of diethyl ether. The flask was stirred with a magnetic stirrer. A nitrogen atmosphere was maintained. 3.0

This small cooling was utilized to maintain the temperature at 25 octene.

mole of l-pentene was dissolved in diethyl ether and added yield of 85 percent.

Identical procedures were utilized for the reaction ofl-octene, l-hexene, cyclohexene, cyclopentene, diisobutylene, styrene and B,fi-diphenylethylene. In all cases, 70-90% yields of the tr-ialkylboron were realized.

Preparation of tricyclohexylboron in Q dimethylether of diethyleneglycol The procedure was similar tothat in the previous preparations indiethyl ether. with two slight modifications. Because of the high boiling point of the solvent, external 35 C. Water was used to wash the solvent from the product. From 0.1 mole of aluminum borohydride complex with the dimethylether of diethylen'eglycol and 0.9 mole of cyclohexene there was obtained 79% yield of tricyclohexylboron, B.P. 130132 C. at 2 mm.

Preparation of tricyclopentylboron in tetrahydrofuran The procedure was similar to those previously described. From 0.1 mole of aluminum borohydride and 0.9' mole of cyclopentene was obtained a 72% yield of tricyclopentylboron, 13.1. .118-120 C. at 2 mm.

Preparation of tri-n-octylboron in inert solvents 0.1 mole of the diethylether complex of aluminum borohydride was added to 200 ml. of n-heptane. To this solution was added 0.9 mole of l-octene, maintaining the temperature below 50 C. The product was distilled under vacuum. There was obtained a 75% yield of tri-n-' octylboron, B.P. 144l46 C. at 2 mm. Similar results were obtained with tetrahydrofuran, anisole, and dimethylether complexes.

The reaction was repeated, but 0.1 mole of triethylamine and 0.1 mole of aluminum borohydride was added totne n-heptane. An 82% yield of tri-n-octylboron was obtained. N-ethylmorpholine, diethylaniline and pyridine yielded similar results.

Finally a preparation was carried out using 0.1 mole of diethylsulfide and 0.1 mole of aluminum borohydride in n-heptane. A yield of of tri-n-octylboron was obtained.

Similar preparations were carried out'in cyclohexane, decalin, benzene, xylene and chlorobenzene as solvent. In all cases the reaction proceeded smoothly. Petroleum fractions free of unsaturates were also satisfactory.

' Preparation of tri-n-octylboron using concurrently formed aluminum borohydride complex A solution of 0.01 mole of aluminum borohydride in 500 ml. of diethyl ether was treated with 1.0 mole of 1- To the vigorously stirred reaction mixture was added 0.2 mole of diborane. The diborane was rapidly absorbed as fast as it could be generated and passed into the ether solution. The reaction mixture was washed with waterand distilled under reduced pressure. There was obtained 103 grams of tri-n-octylboron, the yield being 88 percent.

Thepreparation was repeated using tetrahydrofuran and 0.01 mole of aluminum hydride, prepared from lithium aluminum hydride and aluminum chloride. Again the diborane was rapidly absorbed and the 1- octene was converted almost quantitatively to tri-noctylboron.

Ethylalurninum sequibromide and lithium aluminum hydride with diborane likewise brought about a rapid conversion of olefin to organoboron.

Trimethylalurninurn, 0.01 mole, was added to tetrahydrofuran together with l-octene and diborane passed in. Again a rapid reaction was observed which was complete in less than 60 minutes.

In a separate experiment pure l-octane was treated with diborane at 25 C. In spite of the much higher concentration involved in the use of pure octane, the absorption of diborane in one hour was less than 10% of the quantity estimated for reaction to form tri-n-octyl' boron.

Conversion of butadiene 0.01 mole of aluminum borohydride-diethylether complex was dissolved in diethyl ether. This solution was slowly dropped into a solution of 0.1 mole of butadiene in diethyl'ether. After the reaction was complete, the reaction mixture was treated with water and the ether layer was separated. The ether was removed on a steam bath. The oily organic layer contained both boron'an'd unsaturation. It reacted rapidly with bromine and with oxygen.

Conversion of 3-hexyne 0.1 mole of 3-hexyne was treated in the same manner Conversion of allyldiethylamine Allyldiethylamine, 0.1 mole, was treated with aluminum borohydride-diethyl ether complex. After hydrolysis, there was obtained an oily liquid containing both boron and nitrogen. The compound was soluble in both strong base and in aqueous acid.

Preparation of tri y-ch loroprapylborane Allyl chloride.-A solution of 1.0 mole of allyl chloride in 200 ml. of ethyl ether was treated over a period of an hour with a solution of 0.1 mole of aluminum borohydride-ethyl ether complex in they same solvent. The reaction mixture was allowed to stand for one hour,

then hydrolyzed. The. hydrogen evolved was 0.3 mole, indicating 90% reaction. The ether layer was separated,

the ether stripped off on the steam bath, and the product obtained as an oily liquid containing both boron and chlorine. Oxidation of the product yielded trimethylene chlorohydrin in an overall yield of 75%.

Oleic acid.--A solution of 1.0 mole of oleic acid in 200 ml. of ethyl ether was slowly added over a period of two hours to a solution of 0.5 mole of. aluminum borohydride -ethyl ether in 200 ml. of ethyl ether. Hydrogen was evolved in the initial stages of the reaction as the acid hydrogen of the carboxylic acid reacted with the reagent. After two hours at room temperature, the product was hydrolyzed. Approximately 2.0 moles of hydrogen was evolved, corresponding to the utilization of four moles by the compound. The reaction product was an oily liquid corresponding to the composition (HOC H Q B, which could not be distilled at temperatures up to 200 at 0.1 mm. identified as a mixture of octadecylene glycols,v

mnsst mi i It was concluded that the reagent simultaneously con- Verted the double bond to an organoborane in the usual manner and reduced the carboxyl group to the alcohol stage.

In the same manner, ethyl oleate was converted into the eighteen carbon atom glycol, allyl acetic acid was converted into'the 1,4-glycol, allyl cyanide into the 1,4- tetramethylcne 'aminoalcohol, crotonaldehyde into the corresponding glycol, C H (OH) and mesityl oxide, (CH ),C=CHCOCH into the corresponding glycol, CBH12(OH)2- 7 Thus unsaturated bonds may be hydroborated by the reagent in compounds containing groups reducible by the reagent, provided sufficient reagent is present to both reduce the group and hydroborate the unsaturated linkage. r

The reagent hydroborates unsaturated linkages of all types, including ethylene and acetylene and the various hydrocarbon derivatives of the types: RCH==CH R,CH=CH,, RCH==CHR, R C =CHR, R O=CR,, R-CICH, and R-CsC-R, where R is the same or diflferent hydrocarbon radicals, including aliphatic, aromatic and alicyclic groups which may contain one or more non-reducible groups, or reducible groups provided the reagent is used in suflicient quantity to both I reduce the group or groups and the unsaturated bonds.

This application is a continuation in part of my copending application, Serial No. 630,017, filed December It was oxidized and 24, 1956, now abandoned.

I claim: 1. In a method of preparing an organoboron compound, the step which comprises reacting an aluminum borohydride complex with an organic compound selected from the group consisting of olefins, cycloolefins, dienes, alkynes, arylalkenes, nitroaryl alkenes, nitroalkenes, alkenyl-amines, alkenylethers, alkenyl sulfides, pyridine alkenes, haloalkenes, alkenoics, alkyl alkenoates, alkenyl nitriles and alkenals. thereby forming-an organoboron compound and an aluminum hydride complex, said aluminum borohydride complex being a complex with a complexing agent selected from the group consisting of an ether, a tertiary amine and a thioether which is inert toward said organic compound and said organoboron compound and which is capable of forming said complex with aluminum'hydride.

2. The method claimed by claim 1 wherein the reaction is conducted in an inert liquid carrier.

3. The method claimed by claim 2 wherein diborane is introduced into the liquid carrier to convert the com- 5. The method claimed by claim 2 wherein the inert liquid carrier consists essentially of a saturated hydrocarbon.

, 6. The method claimed by claim 2 wherein the inert liquid carrier consists essentially of a solvent for the aluminum borohydride complex. 7

7. The method of preparing an organoboron compound having the formula R B in which R represents a satu-- rated hydrocarbon radical which comprises reacting an aluminum borohydride. complex with a hydrocarbon containing an olefinic bond thereby forming said organoboron compound and an aluminum hydride complex, said aluminum borohydride complex being a complex with a complexing agent selected from the group consisting of an ether, a tertiary amine and a thioether which is inert toward said organic. compound and said organoboron compound and. which is capable of forming said complex with aluminum hydride.

8. The method claimed by claim 7 wherein the reac-' tion is conducted in a liquid medium whiehconsists essentially of an excessof said complexing agent.

9. The method claimed by claim 7 wherein the reaction is conducted in an inert liquid carrier.

References Cited in the file of this patent UNITED STATES PATENTS 2,858,340 Clark et a1. Oct. 2a, 1950 

1. IN A METHOD OF PREPARING AN ORGANOBORON COMPOUND, THE STEP WHICH COMPRISES REACTING AN ALUMINUM BOROHYDRIDE COMPLEX WITH AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF OLEFINS, CYCLOOLEFINS, DIENES, ALKYNES, ARYLALKENES, NITROARYL ALKENES, NITROALKENES, ALKENYL-AMINES, ALKENYL ETHERS, ALKENYL SULFIDES, PYRIDINE ALKENES, HALOALKENES, ALKENOICS, ALKYL ALKENOATES, ALKENYL NITRILES AND ALKENALS, THEREBY FORMING AN ORGANOABORON COMPOUND AND AN ALUMINUM HYDRIDE COMPLEX, SAID ALUMINUM BOROHYDRIDE COMPLEX BEING A COMPLEX WITH A COMPLEXING AGENT SELECTED FROM THE GROUP CONSISTING OF AN ETHER, A TERTIARY AMINE AND A THIOETHER WHICH IS INERT TOWARD SAID ORGANIC COMPOUND AND SAID ORGANOBORON COMPOUND AND WHICH IS CAPABLE OF FORMING SAID COMPLEX WITH ALUMINUM HYDRIDE. 